This invention relates to the modulation of laser light amplitude for transmitting digital signals over optical fiber transmission systems.
By modulating the amplitude of light inputed into an optical fiber in accordance with the "1"s and "0"s in a digital signal, the digital signal can be transmitted and then recovered at the far end of the fiber by a photodetector which recognizes the changing amplitude of light and converts these changes into an electrical signal for further processing. Generally, a laser diode serves as the transmitting light source, which output is to the transmitting fiber. One way that the light to be transmitted can be converted to a pulsed digital light signal is by directly modulating the electrical current through the laser diode in accordance with the digital signal to be transmitted. Modulating the current through the laser diode modulates the gain of the laser, which translates into a desired pulsed amplitude optical output. Alternatively, a laser diode biased with a fixed current produces a constant amplitude light output, which then can be modulated by an external optical modulator to produce the desired pulsed digital optical signal for transmission.
In the first method, modulation of the current through a laser diode introduces a large phase change to the modulated optical signal to be transmitted. The chirp parameter of the modulated signal, which is defined as the ratio of phase change rate to amplitude change rate, is thus relatively high. The transmission characteristics of an optical fiber normally cause pulse signals to spread or sanear as they are transmitted, which dispersion effect is particularly noticeable when transmitting very high-speed signals over long-haul transmission facilities. This dispersion can make accurate detection at the receiving end difficult. A signal with a high positive chirp parameter will be even further degraded when transmitted and thus be even more difficult to detect accurately at the receiver.
The second method, which does not require modulation of the electrical current through the laser diode, inherently has a low chirp parameter and is thus advantageous for high-speed long-haul transmission. Various prior art approaches can be used to externally modulate the constant light output of a laser diode. In a first approach, a LiNbO.sub.3 Mach-Zehnder modulator (see, e.g., A. H. Gnauck et al, "Dispersion Penalty Reduction Using an Optical Modulator with Adjustable Chirp,"IEEE Photonics Technology Letters, Vol. 3, No. 10, October 1991, pp. 916-918) is used to modulate the light by an interferomic sum of two optical waves originating from the laser diode, wherein each wave is phase modulated by a different amount. In order to function properly, the phase velocity of the electrical signal wave must match the the optical signal phase velocity. Since, however, the electrical signal wave contains spectrum at both high and low frequencies, it is extremely difficult to achieve this goal over a wide frequency range.
A second approach for modulating the light output of a laser diode uses a semiconductor electro-absorption waveguide (see, e.g., F. Devaux et al., "20 Gbit/s Operation of a High-Efficiency InGaAsP/InGaAsP MQW Electroabsorption Modulator With 1.2-V Drive Voltage, IEEE Photonics Technology Letters, Vol. 5, No. 11, November 1993, pp. 1288-1290). This modulator functions by shifting the bandgap of the semiconductor material by an applied electrical field, which is modulated by the digital signal to be transmitted. Disadvantageously, however, modulation with only a positive chirp parameter is possible which, as aforenoted, means that transmission over an optical fiber can only exacerbate the dispersion problem. Furthermore, in some instances this approach is polarization sensitive.
Using an asymmetric Fabry-Perot modulator (AFPM) is a third approach for modulating the light output of a laser diode (see, e.g., L. Buydens et al, "High-Contrast/Low-Voltage Normally On InGaAs/AlGaAs Asymmetric Fabry-Perot Modulator" IEEE Transactions Photonics Technology Letters, Vol. 3, No. 12, December, 1991). The AFPM modulates a constant amplitude light input by modulating the electro-absorption of a semiconductor material within an optical cavity defined by top and bottom reflectors. By modulating the electrical field across the electro-absorption material consisting of a multiple quantum well semiconductor structure disposed between the top and bottom reflectors, the optical absorption of the semiconductor material varies and by that, so does the total reflectivity of the cavity. Specifically, unmodulated constant amplitude light that is incident upon the top reflector is reflected in part and transmitted in the other part through the electro-absorption material. The transmitted through part is reflected by the bottom reflector, transmitted again through the electro-absorption material, and recombined with the original part of the incident light reflected by the top reflector. When the absorption in the electro-absorption material is low, the reflection of the incident light is dominated by the bottom reflectance and the total reflection is high. When the electro-absorption within the material is increased by increasing the applied field, the total reflectivity starts to decrease. When the absorption in each direction in the cavity, A (v), equals 1/2(R.sub.b -R.sub.t), where R.sub.b is the reflectivity of the bottom reflector and R.sub.t is the reflectivity of the top reflector, a cavity matching condition is achieved and the total reflectivity, R.sub.total, equals zero.
Varying the electro-absorption between low (essentially zero) reflectivity at the matching condition voltage and high reflectivity at low or zero voltage across the cavity therefore causes the magnitude of the reflected signal to be modulated in accordance with the modulating electrical field. When the parameters of the cavity, which include the top and bottom reflectivities and the absorption within the electro-absorption material, are chosen properly, the ratio between the amplitude of a "1" and a "0" pulse, defined as the extinction or contrast ratio, is sufficiently large enough to ensure detection at the receiving end. Advantageously, AFPMs can achieve polarization insensitivity and can operate at high-speeds ranging from DC to 40 GHz. Furthermore, their modulated output signal has only a small positive chirp. Disadvantageously, however, the properties of the material composition of the electro-absorption material in prior art AFPMs have been such that they could only operate in wavelength regions between 0.8 and 1.0 .mu.m. Long-haul transmission, however, requires transmission in the lower-loss wavelength region of 1.5 .mu.m. An external modulator that operates at wavelengths in the desirable 1.5 .mu.m region is thus required.
An object of my the asymmetric Fabry-Perot modulator of present invention is to modulate a constant current light input having a wavelength in the region of 1.5 .mu.m at which long-haul optical transmission can take place.
An additional object of the present invention is to produce a modulated light output that has a negative chirp parameter in order to pre-compensate for the accumulation of the positive dispersion effect that normally occurs when the modulated signal is transmitted over optical media.